Is diesel lower CO2 than petrol... really?

[goneape]

And if not, and presuming the government know it, why is the VED so much lower for diesel cars than petrol cars with corresponding performance?

I started writing a complicated question using all sorts of science but as it's been a long afternoon spannering and I've had a few post-grazed-knuckles beers it went somewhat awry...

Essentially what I'm getting at is, during a DPF regen cycle the soot trapped in the DPF gets burned off and in the process some CO2, otherwise unaccounted for in the usual tail-pipe emission tests, must be emitted. Usually it seems in a massive cloud of black smoke out the back of <insert favourite diesel hate car here> all over <name of your local motorway/bypass>.

So... I think my questions are

1. Do the diesel emissions figures take into account the DPF regen emissions?

2. If so, why is it that diesel emits less overall CO2 than petrol - assuming comparable combustion %age - genuine science question, as I'd assumed the deficit ends up as PM10/soot ... which these days gets burned in the DPF

3. If not, why is diesel VED so cheap vs petrol? It seems the cars have the same ultimate effect on the environment.

It's not as if UK has a world leading diesel technology base crying out for investment and promotion (that's Audi), or that north sea crude has a particularly high heavy naptha content. Could be wrong on that last point.

Is it because there's more to be made from taxing a lot of diesel cars benevolently than a few petrol cars swingeingly?

Or is it because there is no science in politics and the politicians have no idea what they're doing?

I think I might know the answer but thought it wise to canvass for opinion...

[numtumfutunch]

Europe - emissions measured by CO2 = diesels

USA - emissions measured by NOX = hybrids

Is that it???

[Disco You]

Do you really think that the government cares about CO2? They are about raising money through high VED.

[Matt UK]

Disco You said:

Do you really think that the government cares about CO2? They are about raising money through high VED.

Agreed.

They must be pissed though that all the car manufacturers have now cracked the code on their bullst EU MPG test cycle and wiped out huge swathes of VED.

Still, the fact that cars don't actually do the numbers means they still get significant revenue through fuel, although I think that in recent years consumption has actually started to decrease YOY.

[b0rk]

CO2 is carbon dioxide which is product of combusting the fuel, one carbon atom combined with two oxygen atoms. A kilogram of diesel contains more carbon than the same kilogram of petrol. However diesel has a higher calorific content than petrol so you need less to produce the same useful output so yes diesel is lower co2 than petrol.

[davepoth]

What comes out of the DPF is mainly carbon, not carbon dioxide. That's what soot is.

[cptsideways]

Diesels also have a higher a BMEP for the same amount of fuel eg more torque output per gram of fuel burnt

[DJP]

Diesels are generally more economical than petrols. Less fuel burnt = less CO2 emitted (in very simple terms).

[10 Pence Short]

The real issue to me, is that MPG figures are no longer considered as guidance of a model's actual capability. Words to the effect are now appearing, to say the figures are purely for comparison purposes.

This leaves consumers unable to accurately gauge the likely running costs of what is the second biggest investment they're ever likely to make.

Unfortunately it's yet another unintended consequence of the EU policies.

[DJP]

10 Pence Short said:

...Words to the effect are now appearing, to say the figures are purely for comparison purposes...

And they're not even much use for that: Just because model A beats model B under highly artificial test conditions, doesn't mean that it will be better for an individual's actual, real world, use.

[Mave]

10 Pence Short said:

The real issue to me, is that MPG figures are no longer considered as guidance of a model's actual capability. Words to the effect are now appearing, to say the figures are purely for comparison purposes.

This leaves consumers unable to accurately gauge the likely running costs of what is the second biggest investment they're ever likely to make.

Unfortunately it's yet another unintended consequence of the EU policies.

But even before the EU policies, this was the case. Manufacturers declared fuel consumption at a number of predetermined conditions, but real fuel consumption varied wildly by comparison.

[emicen]

You get 2.34 kg of CO2 produced per litre petrol burnt
You get 2.68 kg of CO2 produced per litre of diesel burnt

So yes, a litre of diesel produces more CO2, but its higher calorific value means less fuel needs burnt for the same amount of energy.

[edward1]

Yes as you burn less of it to cover the same distance. I suspect it is unlikely that a DPF regen would take place in the EU test cycle as it only covers 11km total, so any fuel efficiency figure or CO2 rating will not include the CO2 liberated as the carbon trapped in the filter is burnt off or the additional fuel used in some vehicles to achieve this.

As for tax I may be wrong but I thought the reason diesel cost more than petrol at the pumps now was down to the extra tax levied due to it being a dirty polluting fuel. this is in obvious contradiction to getting a low VED band for you supposedly efficient low CO2 diesel.

[goneape]

Thanks for the technical replies. I see what you mean but I'm not convinced.

Here are some figures that I've run:

Derv Petrol
2.68 2.34 CO2/kg fuel, kgfrom earlier post, but assumes 100% combustion
0.85 0.72 density, kg/L from the internet, various sources
44300 44400 NHV, kJ/kg ditto
37655 31968 NHV, kJ/L calc. from density
2.278 1.685 CO2/L fuel, kg ditto
16529 18974 kJ/kg CO2

So in a highly theoretical sense petrol is the cleaner fuel as it produces more useful energy per unit of CO2.

Now taking two BMWs, '05-'11 318 petrol vs 318 diesel in poverty trim:

Power = 120 bhp (derv) vs 127
0-60 time = 10.3 secs (derv) vs 9.7
Both engines 1995 cc - data from Parker's

derv gas
50 38 MPG
11.004 8.363 m/L (4.544 L/gallon)
0.2070 0.2015 CO2/mile, kg
129 126 CO2/km, g conversion

In this example using two broadly comparable cars the CO2 emission on a purely theoretical basis is about the same which leads me to conclude that diesels are no more environmentally friendly than petrol engines.

However if account is taken of the imcomplete combustion that presumably gives rise to the soot, then the diesel figures would look better. So that leads me to the conclusion that diesels are lower CO2 than petrol not because of any fuel calorific value or economy arguments but mainly because what they don't chuck out in CO2 they chuck out in soot; and if the DPF regen works by burning off the soot in hot exhaust gas (it seems some do) then the CO2 is released anyway.

QED*

*for about 60 seconds as this is PH!

Edited by goneape on Tuesday 1st October 18:33

[mcford]

The catalytic converter used in a petrol car will generate Carbon Dioxide as it works.

[Superhoop]

The EU drive cycle for fuel economy doesn't include a regeneration of the DPF, in fact any run where a regen does occur is discounted, and the drive cycle repeated, however; the DPF regeneration is then tested seperately, and is then factored into the final test result

So in answer to your original question, yes, the manufacturer's CO2 figures do include the DPF regeneration cycle.